Developmental dysplasia of the hip (DDH) is a common disease of early childhood often called congenital hip dislocation. It results from the disruption of the normal seating of the femoral head in the acetabulum of the hip. Although the incidence is felt to vary from 1 to 4 per 1,000 births, an incidence of up to 13 per 1,000 has been reported. There is a strong genetic component to DDH, with a polygenic component felt to be responsible for the acetabular dysplasia, and a monogenic component felt to be responsible for the lax joint capsule. There is also evidence suggesting a relatively high rate of generalized joint laxity (GJL) in patients with DDH, and defects in collagen have been found in patients with GJL. We hypothesize that the association between DDH and collagen genes is strongest in children with GJL, and if we subgroup patients with DDH based on the presence or absence of GJL we will be able to identify genes associated with DDH in the subgroup with GJL. We also hypothesize that the subgroup of patients with GJL will demonstrate a pattern of inheritance that is close to Mendelian, whereas those without GJL will show a multifactorial inheritance pattern. Transmission disequilibrium testing (TDT) will be used to test the association between DDH and candidate collagen genes in the two subgroups of patients with DDH i.e. those with and those without GJL, and to compare the degree of association of DDH with the candidate genes between the two groups. We will also explore the mode of inheritance of DDH, separately in families of DDH patients with GJL and those without GJL.
